Fuel cells have become increasingly more popular each year since the late 1950's when they were first used to power different devices in space exploration vehicles. Large fuel cells are now used to power cars and buses, and smaller fuel cells power electronic devices, including cellular phones and laptop computers. Thus, fuel cells range in size and can be used for a myriad of different applications. The larger fuel cells typically are designed as large stacks of individual fuel cells that power cars or other vehicles. The smallest fuel cells can be formed on silicon and used to power other silicon based devices, even those fabricated on the same silicon chip as the fuel cell itself. Examples of silicon based fuel cells are disclosed in commonly assigned published patent application Ser. Nos. 2003/0003347 to D'Arrigo et al. and 2004/0142214 to Priore et al., the disclosures of which are hereby incorporated by reference in their entirety.
Fuel cells typically produce electricity from an electrochemical reaction that exists between a fuel gas, such as hydrogen, and oxygen provided from the air. In the larger fuel cell devices or systems, a stack of thin, flat or planar configured fuel cells are layered together. The electricity produced by a single fuel cell is combined with other individual, stacked fuel cells to provide enough power for a vehicle or other application that requires far greater power than an individual fuel cell can provide.
Usually, a fuel cell includes an ion exchange electrolyte formed as a polymer membrane that is positioned or sandwiched between two thin “catalyst” layers operative with anode and cathode electrodes that start the reactions and produce the electricity. Hydrogen is fed to the fuel cell and contacts a first catalyst layer as an anode electrode. Hydrogen molecules release electrons and protons. The protons migrate through the electrolyte to the cathode electrode typically as part of a second catalyst layer and react with oxygen to form water. The electrons separated from the protons at the anode cannot pass through the electrolyte membrane and thus travel around it creating an electrical current.
There are many different types of fuel cells, typically depending on the type of electrolyte positioned between the electrodes. For example, many fuel cells use a polymer electrolyte membrane (PEM) and are termed PEM fuel cells. Other fuel cells can be classified as direct methanol, alkaline, phosphoric acid, molten carbonate, solid oxide, and regenerative fuel cells. Regenerative fuel cell technology also produces electricity from hydrogen and oxygen and generates heat and water as byproducts, similar to other fuel cells, such as the PEM fuel cells. The regenerative fuel cell systems, however, can also draw power from a solar cell or other source to split water formed as a byproduct into both oxygen and the hydrogen fuel using electrolysis. NASA is one group that has been active in developing this technology.
Polymer electrolyte membrane (PEM) fuel cells are the better known and more popular fuel cells because they do not require corrosive fluids, and use a solid polymer as an electrolyte, typically with some type of porous electrode that may contain a platinum catalyst. Usually, the PEM fuel cells receive pure hydrogen from a fuel processor that generates hydrogen in some manner or form a hydrogen storage tank or other storage system. The PEM fuel cells typically operate at low temperatures, around 80° C. (176° F.), which allows them to start quickly with less warm-up time. This results in reduced wear, increased durability, greater power per pound of fuel gas, and overall better operation. Usually some type of mobile metal catalyst is operative with the anode, for example, platinum, and separates the hydrogen's electrons and protons. Another catalyst could be operative with the cathode to aid in the reaction using oxygen and air.
In many types of fuel cells, storing hydrogen for sustained fuel cell operation is a drawback and different techniques have been devised for generating and/or storing hydrogen for sustained fuel cell operation. For example, fuel cells store hydrogen chemically using a metal hydride or carbon nano-tubes, which are microscopic tubes of carbon, for example, two nanometers across. Whatever type of hydrogen storage or generation is used, however, what distinguishes the fuel cell particularly is the use of an ion exchange electrolyte, such as a polymer electrolyte membrane (PEM), operative as a proton exchange membrane. These types of membranes are typically formed as an ion-exchange resin membrane and can be applied as a very thin film, sometimes even poured or wiped on. PEM is usually made from perfluorocarbonsulfonic acid, sold under the tradename “Nafion,” phenolsulfonic acid, polyethylene sulfonic acid, polytrifluorosulfonic acid, and similar compounds. Other examples may include those compounds discussed in the incorporated by reference '347 and '214 patents. Some porous carbon sheets are impregnated with a catalyst, such as platinum powder, and placed on each side of this resin membrane to serve as a gas diffusion electrode layer. This structure and assembly is usually termed a membrane-electrode assembly (MEA) by many skilled in the art.
A flow divider is often operative at the electrodes and anode and cathode. A flow divider at the anode forms a fuel gas passage on one side of the MEA. An oxidizing gas passage can be formed on the other side of the MEA using a flow divider. Distribution plates, separation plates, or other assemblies, including silicon structures as disclosed and claimed in the above-identified '214 and '347 incorporated by reference published patent applications could be operative as flow dividers.
Fuel cells are also becoming increasingly desirable as substitutes for standard AAA, AA, C and D sized dry cell batteries. Many prior art fuel cell devices designed with stacks of individual PEM fuel cells have not been found adequate for application as dry cell battery substitutes. The fuel cells generate water and do not have efficient control of fuel gas into the fuel cell. The generated water could create a problem, as well as storage or generation of any hydrogen gas to enable sustained fuel cell operation. Also adequate control over fuel gas input to the fuel cell is desirable to increase its efficiency and reduce its waste.